This invention addresses problems encountered in the bottom pouring of liquid titanium (or titanium alloys).
The high level of chemical reactivity of liquid titanium or liquid titanium alloys leads to chemical reaction between such liquid and all oxide, oxysulfide, sulfide, boride or other compound ceramics. Further, all metals having a melting point higher than titanium will dissolve in liquid titanium. In short, there is no known inert containment vessel material other than titanium itself to hold molten titanium or titanium alloys. In keeping with this limitation, titanium and titanium alloys are melted by a technique called cold hearth or skull, melting.
In this technique, pieces of solid titanium are placed in a cooled metal hearth, usually made of copper, and melted in an inert atmosphere using a very intense heat source, such as an arc or plasma. During the melting process a molten pool will form initially on the interior and top surface of the charge of metal while the titanium adjacent the confining wall of the copper hearth remains solid. The "skull" of solid titanium, which develops, contains the liquid titanium metal free of contamination. The technique is used in conjunction with a consumable titanium or titanium alloy electrode for virtually all titanium primary melting and casting at the present time.
In the preparing of titanium castings, melting is generally accomplished by consumable arc melting and liquid metal so generated is poured over the lip of a skull crucible into a mold. Inherent in the act of pouring over a lip is the characteristic that a thin liquid cross section is maintained at the lip. Heat loss from the liquid as it passes over the lip will reduce the superheat of the liquid metal typically leading to the formation of a solid-liquid mixture rather than the desired liquid. Although over-the-lip pouring can be tolerated in the preparation of castings, in those applications in which a lower liquid flow rate, or at the least, a steady liquid flow rate is required, (e.g. rapid solidification) the only promise for a variable solution appears to lie in bottom pouring from a cold hearth melting system through a nozzle.
The major drawbacks of cold hearth melting and bottom pouring of reactive metals are (a) the problem of melt freeze-off in the nozzle and (b) erosion of the nozzle material by the liquid metal.
Systems have been described in the literature utilizing cold hearth arc melting in a thermally conductive hearth with bottom-ejection of the liquid metal through a nozzle insert. The nozzle material typically employed has been copper or brass, which are considered good thermal conducting materials. Graphite has also been mentioned as a nozzle material. Nozzles made of thermally insulating material also have been suggested for such a system. None of the attempts described to date have been successful in providing the requisite control of liquid flow rate and/or minimal erosion and/or minimum melt contamination.
It has, therefore, been an object of this invention to discover a nozzle material having adequate resistance to erosion and a cold hearth and nozzle configuration enabling the successful bottom pouring of liquid titanium and titanium alloys.
The term "effective diameter" as used herein is the diameter of the circle that can be inscribed in the particular planar shape (e.g. a square) in question.
"High" thermal conductivity implies a value in excess of about 80 watts/meter .degree.C. at 700.degree. C.